The invention relates to a method and apparatus for temporarily retaining the lateral walls of a trench, after the latter has been dug in ground which is especially, but not exclusively, an underwater bed consisting of materials having poor cohesion, such as sands, silts, muds or clays, in order to allow a pipe, which is preferably substantially rigid, such as an oil pipe or a pipeline having a large diameter, and which comprises an internal metal wall enclosed, if necessary, by a protective concrete sheath, to be laid in the trench.
To protect pipes, and especially oil pipes or pipelines, from the harmful effects of underwater currents or from the swell of the sea, and also to prevent marine anchors or trawl nets for fishing from hooking onto these pipes, attempts have been made for many years to embed these pipes in underwater beds. This has been carried out by placing these pipes in trenches dug in the underwater beds. Experience has shown that it is very difficult, indeed virtually impossible, in the present state of the art, to place an oil pipe in a trench previously dug in an underwater bed. In contrast, numerous devices, both mechanical and hydraulic, have been developed to allow a trench to be dug under a pipe previously placed on the underwater bed, the pipe coming to rest, by bending under the effect of its own weight, on the bottom of the trench at a certain distance behind a device for digging the trench, which device is moved along under the pipe.
Amongst the mechanical means developed for this purpose are some with which the material forming the underwater bed under the pipe can be dredged or cut away.
It is also known to move under the pipe a plough having a ploughshare which pushes back the material of the underwater bed laterally to both sides relative to the direction of advance of the plough. Furthermore, the hydraulic devices used for the same purpose permit a hydraulic cutting away of the underwater bed under the pipe using, for example, a plurality of nozzles for ejecting a liquid, such as water, at a low quantity discharge rate but at high pressure, the nozzles being supported by a metal gantry which is moved on the pipe.
Although the use of these different known devices and methods sometimes achieve good results when the underwater bed consists of materials sufficiently cohesive that a trench having vertical walls, approximately vertical walls or, at least, steeply inclined walls, is sure to be stable, the same does not apply when the underwater bed is sandy, muddy or consists of any other material that is pulverulent or has inadequate cohesion, because the lateral walls of the trench may collapse and the trench may fill in, at least partially, before the pipe is suitably positioned on the original base of the trench.
In the latter case, when using the above-mentioned devices, in order to prevent the immediate, or almost immediate, filling in of the trench a depression or valley of great size has to be made in the underwater bed. On the one hand, that makes it necessary to move very great volumes of material and, on the other hand, it does not achieve the desired protection for the pipe unless the depression is filled in on either side of the pipe and, if necessary, above the pipe, once the pipe is in place on the bottom of the depression, by replacing a major portion of the materials removed when digging, which substantially increases the cost of the operation.
Attempts have also already been made to tow behind the digging device cofferings or formers having two rigid sides for temporarily supporting the lateral walls of the trench. In order, however, to allow the walls to collapse only after the descent and positioning of the pipe in the trench, these sides must extend over a very great distance and, for this reason, it is very difficult, if not impossible, to move them.
To overcome these various disadvantages, it has already been proposed to embed a pipe, such as an oil pipe or pipeline, in an underwater bed consisting especially of materials having poor cohesion by applying under this pipe the method for digging trenches known by the terms "mud" trenches or trenches "having moulded walls", which method comprises replacing the materials extracted from the trench being dug by mud as the trench advances, so that the mud constantly exerts sufficient pressure on the lateral walls of the trench, which walls are approximately vertical or steeply inclined, that these walls do not collapse before the pipe is positioned on the bottom of the trench, the general term "mud" designating any of those wellknown media which are obtained by mixing, for example, argillaceous materials and anti-flocculants, such as those marketed under the name Bentonite, or other thixotropic products and which remain constantly deformable and have only poor cohesion.
The method for embedding a pipe in an underwater bed which is the subject of that proposal consists, therefore, in laying the pipe on the underwater bed, then digging a trench under the pipe and replacing the materials removed from the trench which is being dug by mud, as the trench advances, in such a manner that the lateral walls of the trench are prevented from collapsing, and, finally, in the pipe's positioning itself on the bottom of the trench.
If the underwater bed is argillaceous, it is advantageous, according to that proposal, that the argillaceous materials removed while the trench is being dug are filtered and, in a mixing device which is moved together with the moving arrangement involved in forming the "mud" trench, are suitably treated in such a manner that these argillaceous materials are mixed with anti-flocculants and then re-injected into the trench in the form of mud, this recycling of materials removed from the underwater bed making possible considerable savings in the amount of mud that must be conveyed to the site.
It is also proposed that the materials removed should be mixed in the trench and that water and anti-flocculants should be added in situ in such a manner, that the mud filling the trench is produced directly in the latter.
It should be noted, however, that the use of these economical methods is only possible provided that the underwater bed consists of clays having appropriate physicochemical properties, which is not always the case. Furthermore, the mechanical means to be provided in order that the materials available on the site can be re-used are costly and delicate to operate so that it appears preferable to convey to the site, in every case, the required quantities of, for example, Bentonite which is used in association with the liquid medium available on the site.
Under these conditions, very large quantities of dry products have to be moved, which is a further not substancial disadvantage.
This disadvantage is also encountered when the method of "mud" trenches or trenches "having moulded walls" is used on a land site to ensure that the lateral walls of trenches dug in an emerging soil are retained.